Instrument Panel, Gauges and Warning Indicators: Description and Operation
How the Circuit WorksWhen the ignition switch is on ON (II) or START (III), battery voltage is supplied through fuse 13 to the gauges in the gauge assembly. The gauge circuit is grounded at G401.
Speedometer and Odometer
The odometer and speedometer drive circuits receive pulses from the vehicle speed sensor (VSS). The pulse rate increases as the car accelerates. The frequency and duration of these input pulses are measured and displayed by the speedometer, odometer, and tripmeter.
Tachometer
The tachometer drive circuit receives pulses from the ignition control module (ICM) in the distributor assembly. The solid-state tachometer then displays these pulses as engine speed. For each 200 pulses per minute from the ignition control module (ICM), the tachometer displays 100 RPM.
Engine Coolant Temperature Gauge and Fuel Gauge
The engine coolant temperature gauge has two intersecting coils wound around a permanent magnet rotor. Voltage applied to the coils, through fuse 13, generates a magnetic field. The magnetic field, controlled by the engine coolant temperature sending unit, causes the rotor to rotate and the gauge needle to move. As the resistance in the sending unit varies, current through the gauge coils changes, pulling the gauge needle toward the coil with the stronger magnetic field. The fuel gauge works the same way.
The resistance of the sending unit for the engine coolant temperature gauge varies from about 137 ohms at low engine temperature to between 30 - 46 ohms at high temperature (radiator fan running)
WARNING: Do not smoke while working on the fuel system. Keep open flame away from the work area. Drain fuel only into an approved container.
The resistance of the sending unit for the fuel gauge varies from about 2 - 5 ohms at full, to about 110 ohms at empty. When you turn the ignition switch to LOCK (0), the gauge remains at the last reading until you turn the ignition switch to ON (II) or START (III) again.